1. Field of the Invention
The present invention relates to a motor drive control apparatus configured to control operations of an AC motor, as well as to a corresponding motor drive control method.
2. Description of the Prior Art
One proposed motor drive control apparatus disclosed in Japanese Patent Laid-Open No. 2007-159368 is equipped with an inverter for driving an AC motor and, in response to selection of a first control method of applying a square wave voltage to the AC motor, performs feedback control to adjusting the phase of the square wave voltage according to a torque difference relative to a torque command value. This prior art motor drive control apparatus estimates an output torque of the AC motor based on the motor electric current and the rotational position of a rotor in the AC motor, and specifies the phase of the square wave voltage according to a difference between the estimated output torque and the torque command value. The voltage conversion in the inverter is controlled to apply the square wave voltage having the specified phase to the AC motor. Another proposed motor drive control apparatus disclosed in Japanese Patent Laid-Open No. 2004-222448 includes a rotational position sensor arranged to detect a rotational position of a rotor in a motor, an inverter arranged to have multiple different phase arms and supply an ac power to the motor, and a controller of performing square wave control of the motor to provide a switching timing for switching over switching commands to the multiple different phase arms in response to every change of the rotational position by a preset angle as a division of one rotation of the rotor. This prior art motor drive control apparatus specifies one cycle time required for one rotation of the rotor based on a detection value of the rotational position sensor and a time elapsed since a reference time when the detection value of the rotational position sensor shows a reference value corresponding to a preset reference rotational position. The detection value of the rotational position sensor is corrected at each switching timing, based on each elapsed time measured whenever the detection value of the rotational position sensor reaches a value corresponding to each switching timing, one cycle time, and an expected rotational position corresponding to each switching timing. The controller changes over switching commands, based on the corrected detection value.
The square wave control method adopted in the prior art motor drive control apparatus has a lower control accuracy (poorer control response) than the sine wave PWM control method, but effectively enhances the output of the AC motor and improves the energy efficiency with reduction of a potential copper loss or a switching loss. Expansion of the scope of application of the square wave control method in the working range of the AC motor is expected to further improve the energy efficiency and the performance of the motor drive control system including the AC motor. The feedback control for phase adjustment in application of the square wave control method may, however, not ensure output of a maximum torque, which is expected to be output from the AC motor in a certain condition. There is accordingly still a room for improvement in the prior art motor drive control apparatus.